This invention relates generally to accelerometers. More particularly, it relates to an accelerometer for measuring acceleration in the plane of a substrate within which the accelerometer is fabricated.
Accelerometers are used in a number of automotive, aerospace and sensing applications. Accelerometers are now commonly made from silicon or similar materials using micromachining techniques. FIG. 1 shows a typical accelerometer according to the prior art.
A proof mass 4 is suspended on the free end of a flexure 6. The flexure is supported by a substrate 9. The flexure 6 has a large height 8 and small width 10 so that it is preferentially sensitive to acceleration in an X-direction 12. A strain gauge 13 is located on the flexure 6 so that acceleration is measured. The deflection of the flexure can also be measured capacitively.
Prior art accelerometers sometimes do not have a high enough sensitivity for certain applications. The strain sensitivity can be increased by narrowing the flexure. However, this causes problems because this reduces the area available for the strain gauge. The proof mass can be increased, but this increases the overall size of the device, reducing the number of devices per wafer and increasing the unit cost.
Accelerometers also suffer from xe2x80x98DC-offsetsxe2x80x99 due to residual strain in the substrate, to stress induced by packaging techniques, and other fabrication variations (e.g. ion implantation). Strain caused during packaging is particularly burdensome because trimming resistors inside the package cannot be adjusted to compensate. This results; in a relatively wide range of operating characteristics for packaged devices. It would be an advance in the art to reduce DC-offsets in the operating characteristics of micromachined accelerometers.
Also, accelerometers suffer from thermal offsets caused by temperature dependence of the strain gauge and mechanical properties of the flexure. It would be an advance in the art to provide improved methods and structures for reducing temperature dependent effects.
Another important consideration in accelerometer design is the directional sensitivity. For some applications it is important for the accelerometer to only be sensitive to acceleration in a particular direction. The device of FIG. 1, for example, is designed to sense accelerations only in the X-direction 12. However, the device is also sensitive to accelerations in other, orthogonal directions (e.g. acceleration perpendicular to substrate). The device of FIG. 1 has relatively low directional sensitivity. It would be an advance in the art to provide a micromachined accelerometer with improved directional sensitivity.
Yet another consideration in accelerometer design is protecting the flexure from breaking when subjected to very high accelerations. For example, a device can experience an acceleration of 10,000 G""s if dropped from a table to a hard floor. It is particularly difficult to provide both high sensitivity, and high tolerance to G-shock.
U.S. Pat. Nos. 4,981,552 and 4,969,359 to Mikkor disclose micromachined accelerometers in silicon made using anisotropic wet etching techniques. The accelerometers are capable of sensing accelerations in three dimensions using three high aspect ratio flexures oriented in three orthogonal planes. A problem with the devices of Mikkor is that, since a wet etch is used, the flexures have very sharp corners. Sharp corners concentrate strain and therefore render the flexures susceptible to breaking under high accelerations. Also, the devices of Mikkor are relatively difficult to manufacture.
U.S. Pat. No. 5,395,802 to Kiyota et al. discloses a micromachined accelerometer for measuring acceleration perpendicular to the substrate. Kiyota does not teach accelerometers that measure acceleration parallel with the substrate.
U.S. Pat. No. 4,653,326 to Danel et al. discloses an accelerometer for measuring acceleration parallel with the substrate. The accelerometer uses a high aspect ratio flexure oriented perpendicular to the substrate surface.
U.S. Pat. No. 5,151,763 to Marek et al. discloses an accelerometer for measuring acceleration parallel with the substrate. The accelerometer is made by appropriately doping the substrate and using a dopant sensitive etch to release the flexures.
U.S. Pat. No. 4,891,984 to Fujii et al. discloses an accelerometer for measuring accelerations in three orthogonal directions using flexures oriented in three orthogonal planes.
Accordingly, it would be an advance in the art to provide an accelerometer having reduced sensitivity to temperature changes and substrate strain. Also, it would be an advance in the art to provide an accelerometer having higher directional sensitivity than provided in prior art devices.
Accordingly, it is a primary object of the present invention to provide a micromachined accelerometer that:
1) has a high acceleration sensitivity;
2) is relatively insensitive to substrate strain and strain caused by packaging;
3) is relatively insensitive to errors caused by temperature changes;
4) is sensitive to acceleration in a designated direction and is insensitive to acceleration in orthogonal directions;
5) is caged so that it is protected from G-shock;
6) has a relatively high resonant frequency;
7) is relatively insensitive to rotational acceleration;
8) is easily manufactured at low cost;
9) has a low noise floor compared to signal level.
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
These objects and advantages are attained by a micromachined accelerometer having a base layer, an unreleased portion with a hole, a strain-isolation pedestal, a flexure and a proof mass. The unreleased portion is disposed on top of the base layer. The strain-isolation pedestal is attached to the unreleased portion extending into the hole. The flexure is attached to the pedestal, and the proof mass is attached to the free end of the flexure. The flexure has a high aspect ratio and is oriented to bend in the plane of the base layer. The flexure is narrower than the pedestal. The strain-isolation pedestal serves to isolate the flexure from residual stress in the base layer and surrounding unreleased portion. The pedestal, flexure and proof mass are released from the base layer so that the proof mass is free to move when the flexure bends.
The present invention includes certain preferred size dimensions for the strain-isolation pedestal as a function of the flexure thickness and other dimensions. For example, the strain-isolation pedestal preferably has a width W in a range 2F less than W less than 4T, where F is a thickness of the flexure, and where T is a thickness of the strain-isolation pedestal. Also preferably, the strain-isolation pedestal has a length D in a range 0.5T less than D less than 2W, where T is a thickness of the strain-isolation pedestal, and where W is a width of the strain-isolation pedestal.
The flexure can have an aspect ratio in the range of about 3-30. The flexure can have a thickness in the range of about 2-8 microns. Preferably, joints between the flexure and pedestal, and between the flexure and proof mass are rounded and have a radius or curvature of at least 0.25 F. Alternatively, the joints have a radius of curvature of at least 1 micron.
Preferably, the flexure has a piezoresistor located in a sidewall of the flexure. Also preferably, the piezoresistor is confined to the upper xc2xd of the flexure. More preferably, the piezoresistor is confined to the upper {fraction (1/10)} of the flexure.
The present invention also includes a Wheatstone bridge circuit having two accelerometers and two reference accelerometers. The accelerometers have relatively large proof masses, and the reference accelerometers have relatively small proof masses. The accelerometers and reference accelerometers may or may not have strain-isolation pedestals. Each accelerometer and each reference accelerometer has a single piezoresistor. The four piezoresistors are connected in a wheatstone bridge so that high sensitivity to, linear acceleration is provided. The piezoresistors can also be connected in a wheatstone bridge so that high sensitivity to rotational acceleration is provided. Preferably, the accelerometers and reference accelerometers are oriented antiparallel in pairs.
The present invention also includes an accelerometer having a base, an unreleased portion disposed on the base an having a hole, a strain-isolation pedestal attached to the unreleased portion, a flexure attached to the pedestal and a proof mass attached to the flexure. The pedestal, flexure and proof mass are released from the base. The accelerometer has two piezoresistors in the opposing sidewalls of the flexure. The piezoresistors are electrically connected in series. A center tap connection is electrically connected to the point where the piezoresistors are connected. Preferably, the center-tap connection is provided by a heavily doped path extending along a top of the flexure. This arrangement provides for high sensitivity to accelerations perpendicular to the flexure, and high rejection of accelerations in other directions. The strain-isolation pedestal in this embodiment is optional.
Preferably, the piezoresistors are confined to the upper xc2xd of the flexure.
The accelerometer having two piezoresistors can be electrically connected with a second such accelerometer in a Wheatstone bridge circuit. The wheatstone bridge circuit has certain sensitivity advantages.